A scanning transmission electron microscope is used to analyze the structure of a material. In the scanning transmission electron microscope, the surface of a sample is scanned with a convergent electron beam, and the arrangement of atoms can be observed with high resolution by detecting the electrons transmitted through the sample.
FIG. 1 is a diagram schematically illustrating a scanning transmission electron microscope according to a prior art example.
The electron beam B entering the sample S along the optical axis of the electron microscope passes through the sample S where the electrons are scattered by the atoms composing the sample S. Then, electrons that have been scattered at high angles with respect to the optical axis are detected by an annular first detector 126. While the surface of the sample S is being scanned with the convergent electron beam B, the electrons scattered at high angles are detected by the first detector 126 on a per-scan basis, and a dark-field image of the sample S is obtained by converting the detected signal into a visible image as a function of the scanning position of the electron beam.
The electrons scattered at high angles by the sample S and detected by the first detector 126 are those scattered primarily by thermal diffuse scattering by the atoms composing the sample S. The scattering center of the thermal diffuse scattering is the atomic center, and its scattering intensity increases with increasing atomic number. The method of capturing a dark-field image using a scanning transmission electron microscope in this manner is called HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy).
Electrons scattered at smaller angles than the electrons detected by the first detector 126 are detected using a circular second detector 127. A bright-field image is obtained by converting the signal detected by the second detector 127 into a visible image. In this way, using the scanning transmission electron microscope, not only a dark-field image but also a bright-field image can be captured.
With the above HAADF-STEM method, high-resolution atomic images can be obtained since the captured atomic images are not sensitive to the defocus value nor are they sensitive to the thickness of the sample. Furthermore, since the brightness of the atomic image appearing at each atomic position depends on the atomic number, heavier atoms having higher atomic numbers are easier to detect.
On the other hand, with the HAADF-STEM method, it may be difficult to detect light atoms having low atomic numbers, because in that case the number of electrons scattered by thermal diffuse scattering is small. In particular, in the case of a sample containing heavy atoms and light atoms having different atomic numbers, it is difficult to capture images of such heavy atoms and light atoms with good contrast.
In view of this, a method has been proposed that captures images of heavy atoms and light atoms with good contrast by using a scanning transmission electron microscope.
FIG. 2 is a diagram schematically illustrating a scanning transmission electron microscope according to another prior art example.
The scanning transmission electron microscope depicted in FIG. 2 includes an annular detector 129 for detecting electrons. The annular detector 129 is similar in structure to the first detector 127 depicted in FIG. 1, except that the former has a vacant circular center.
By capturing a bright-field image using the annular detector 129 depicted in FIG. 2, good contrast images of heavy atoms and light atoms can be captured simultaneously.    Japanese Laid-open Patent Publication No. 2010-257883    Japanese Laid-open Patent Publication No. 2009-514141    Non-patent literature, S. J. Pennycook, et al., Phys. Rev. Lett. 64 (1990)938    Non-patent Publication, S. D. Findlay, et al., App. Phy. Lett. 95 (2009)191913